GB2406010A

GB2406010A - Optical transceiver with a two-wire interface for digital control data

Info

Publication number: GB2406010A
Application number: GB0321269A
Authority: GB
Inventors: Technologies Inc Agilent; Sonia Ashmore; Jeremey Crouch
Original assignee: Agilent Technologies Inc
Priority date: 2003-09-11
Filing date: 2003-09-11
Publication date: 2005-03-16
Anticipated expiration: 2023-09-11
Also published as: GB2406010B; GB0321269D0

Abstract

The system uses optical signals modulated by both data content and digital control data (tone signals). This allows an optical transceiver in which tone signals are passed to and from the transmitter and the receiver using a 2-wire interface 500, resulting in a transceiver of reduced size.

Description

OPTICAL TRANSMITTER AND RECEIVER

The invention relates to an optical transmitter and receiver for use in optical communications networks, and in particular to an integrated transceiver comprising a transmitter and a receiver.

It is commonplace to use optical transmitters, such as lasers, to transmit light signals over optical fibres.

The transmitted light is modulated in order to transmit data but it is also known to superimpose a low frequency electrical signal onto the optical signal in order to enable the further transmission of control signals for network elements and network control systems.

According to a first aspect of the invention there is provided a optical transmitter comprising an optical source, a first modulation signal being applied to the optical source to modulate the output of the optical source, the first modulation signal representing content data to be transmitted by the optical transmitter and a second modulation signal being applied to the optical source to modulate the output of the optical source, the second modulation signal representing control data to be transmitted by the optical transmitter, the optical transmitter being characterized in that the control data comprises a digital signal. The optical transmitter may further comprise a two-wire interface to apply the digital control data to the optical source.

According to a second aspect of the invention there is provided an optical receiver comprising an optical - 2 detector, the detector receiving an optical signal comprising a first modulation signal representing content data and a second modulation signal representing control data, the optical receiver recovering the content data and the control signal from the received optical signal, the optical receiver being characterized in that the control data comprises a digital signal. The optical receiver may further comprise a two-wire interface to transmit the digital control data from the optical receiver.

According to a third aspect of the invention there is provided an optical transceiver comprising an optical transmitter as described above and an optical receiver as described above.

Whilst the use of digital control signals per se is well known, in the present invention the use of the digital control signals in contrast to the conventional analogue tone signals enables the control signals to be sent over a conventional two-wire interface. This then allows the tone terminals to be removed from the transmitter and the receiver, allowing transceiver modules to be reduced in size and increasing the module density in equipment racks.

The use of digital control signals and a two-wire interface also enables the tone functions to be managed and controlled via the systems that are used to manage other aspects of module functionality.

The invention will now be described, by way of example only, with reference to the following Figures in which: Figure 1 shows a schematic depiction of a known - 3 optical transceiver; and Figure 2 shows schematic depiction of an optical transceiver according to the present invention.

Figure 1 shows a schematic depiction of a known optical transceiver 10 comprising a transmitter module 20 and receiver module 30. The transmitter module 20 comprises laser 21, 22, laser modulator 23, transmitter control means 24, first and second data input ports 25, 26 and tone input port 27. The receiver module 30 comprises detector 31, pre- amplifier 32, post amplifier 33, low-pass filter 34, amplifier 35, first and second data output ports 36, 37 and tone output port 38. The transceiver also comprises a two wire serial interface 50 comprising clock 52 and data 54 input/output ports. Such interfaces are known for transmitting control data and reporting back measured data to control systems, for example laser diode temperatures, power measured by optical monitors, etc. In Figure 1, the serial interface 50 is shown to interface with the laser 21 and the detector 31 but it will be understood that the serial interface may also interface with other components in the transceiver.

In use, data for transmission is presented at the first and second data input ports 25, 26 and is received by the laser modulator 23. The laser modulator determines the output of the laser 21 that is launched into first optical fibre 40 and is controlled by transmitter control means 24. The transmitter control means sends two signals to the laser modulator in order to control the behaviour of the laser modulator. The first signal controls the bias current applied to the laser, which determines the average - 4 optical power outputted by the laser, and the first signal is determined in relation to the signal sent from the laser monitor 22. The laser 21 is configured such that a small fraction of the optical output of the laser is detected by the laser monitor in order that the performance of the laser can be monitored and the bias current adjusted as necessary by the transmitter control means in order to achieve the desired average output power from the laser. The second signal controls the amplitude of the modulation current applied to the laser. This amplitude is maintained by the transmitter controller in a pre-determined ratio to the bias current in order to maintain the required difference in optical power between the 1 and 0 level used in data transmission.

The transmitter control means is also connected to the tone input 27, to which a low frequency analogue signal is applied. The transmitter control means superimposes this low frequency signal onto the amplitude signal to the laser modulator such that the 1 level of the optical output of the laser is also modulated with the low frequency tone signal.

Conventionally, the light transmitted over the first optical fibre 40 is received at a second optical transceiver (not shown), which will transmit data, which is received by transceiver 10 from the second optical fibre 45. The optical signal is received by the detector 31 and is converted into an electrical signal. The electrical signal passes through a pre-amplifier 32 and a post-amplifier 33 before the electrical signal is presented at the first and second data output ports. The data may then be transmitted to a terminal, such as a computer or a telephony switch, or it may be de- multiplexed prior to sending the data to one or more terminals.

In order to access the tone signal added to the data signal a fraction of the electrical signal is routed to a low-pass filter 34 to recover the tone signal, which is then amplified by amplifier 35 and can be accessed from tone output port 38.

Figure 2 shows schematic depiction of an optical transceiver 100 according to the present invention comprising a transmitter module 200, receiver module 300 and control module 400. The transmitter module 200 comprises a laser 210, a laser modulator 220 and first and second data input ports 230, 240. The receiver module 300 comprises detector 310, pre- amplifier 320, post-amplifier 330 and first and second data output ports 340, 350. The control module 400 comprises processing means 410, analogue to digital converter (ADC) 420, low-pass filter 430, laser monitor 440, digital to analogue converter (DAC) 450, function generator 460, clock input/output port 540 and data input/output port 520.

In use the transceiver 100 operates in a generally similar manner to the transceiver described above with reference to Figure 1. In particular, an electrical data signal will be applied to first and second data input ports and this data will be used by the laser modulator to modulate the optical output of the laser, which will be launched into first optical fibre 40. Similarly, optical signals - 6 received at the detector 310 will be converted into the electrical domain and will be amplified by the preamplifier and the post-amplifier before being presented at the first and second data output ports.

The present invention is distinguished over the prior art by the function and structure of the control module 400.

The laser monitor 440 measures a fraction of the light emitted by the laser and a signal representing this is transmitted to ADC 420. The processing means 410 then generates a first digital signal that is sent to the DAC 450, which converts the first digital signal into an analogue bias current signal that can be used to control the laser modulator 220. The processing means then also generates a second digital signal based on a pre-defined algorithm, which represents the modulation amplitude control signal. This in turn is sent to a DAC, which converts the second digital signal into an analogue modulation current control to the laser modulator.

The clock input/output port and data input/output port 520, 540 form a conventional two-wire serial interface 500 that can be used to transit commands to the processing means. The processing means can then interpret the commands, for example by comparing them against a look-up table, to determine one or tone signal parameters such as frequency, duty cycle, gain, etc. The processor then determines an appropriate signal, which added to the modulation current control and passed to the function generator and subsequently to the DAC in order to provide the required modulation current that is used by the laser modulator to add the modulation tone to the optical output - 7 of the laser.

The optical signal received by the detector 310 includes tone modulation and this information can be recovered by the control module 400. A portion of the electrical data signal is fed, having been amplified by the pre-amplifier 320, to the ADC 420 via a low-pass filter 430. The ADC sends a digital signal to the processing means 410 in response to the analogue signal received from the low-pass filter and the processing means interprets this digital signal to determine the transmitted tone signal. A signal may then be transmitted via the data input/output port and clock input/output port so that the received tone signal may be monitored and the information derived therefrom used to provide feedback.

In a preferred embodiment the processing means comprises 410 comprises memory means 415 that is capable of storing a number of tone parameters for both transmitted and received signals. The operator of the transceiver can use the data input/output port and clock input/output port, which may be a conventional two-wire serial interface, to write tone parameter values into the associated memory locations. When tone parameter values are detected within received signals then these values may be written into those associated memory locations. These parameters may be reported to a monitoring system or corrective action may be undertaken if, for example, a parameter exceeds a defined threshold value. - 8

Claims

1. An optical transmitter (200) comprising an optical source (210), a first modulation signal being applied to the optical source to modulate the output of the optical source, the first modulation signal representing content data to be transmitted by the optical transmitter and a second modulation signal being applied to the optical source to modulate the output of the optical source, the second modulation signal representing control data to be transmitted by the optical transmitter, the optical transmitter being characterized in that the control data comprises a digital signal.

2. An optical transmitter according to claim 1, the optical transmitter (200) further comprising a two-wire interface (500, 520, 540) to apply the digital control data to the optical source.

3. An optical receiver (300) comprising an optical detector (320), the detector receiving an optical signal comprising a first modulation signal representing content data and a second modulation signal representing control data, the optical receiver recovering the content data and the control signal from the received optical signal, the optical receiver being characterized in that the control data comprises a digital signal.

4. An optical receiver according to claim 3, the optical receiver (300) further comprising a two-wire interface (500, 520, 540) to transmit the digital control data from the optical receiver.

5. An optical transceiver (100) comprising an optical transmitter (200) according to either claim 1 or claim 2 and an optical receiver (300) according to either claim 3 or claim 4.